You don't have to tho? You can simply ask her if it would be okay with her and she will be, it's not like "super good guy", but it is respectful of her and a bit of a "We are both okay with it and we need the info" thing which she then very much enjoys, which then can be taken as a reason to continue sharing her later on.
You can of course also be a asshole and just do it without her consent, but the option to be nicer about it is very much there...
Like, yes, it doesn't start out with her wanting to have Sex with others and you wanting to share her so you both decide on it because you want to do it, its more born out of the situation you are in and once it happens you both find out you liked it, I find that a good natural introduction of the sharing aspect.